Pressure swing adsorption process

ABSTRACT

A pressure swing adsorption process wherein a constant flow of feed gas and product gas is maintained to and from a pressure swing adsorption installation. A part of repressurization is carried out by a product gas split-off, in three stages, with an additional pressure equilibration step between two adsorbers during the second of the three stages, thereby cyclically fluctuating one of the number of adsorbers receiving feed gas during some period. In addition, either combined herewith or not, in case of the use of a higher purge gas pressure, is the return to a regenerated adsorber of a least impure part of the offgas, driven by its initially higher pressure. The result is an increased product recovery efficiency and an improved flexibility of functioning of the installation.

BACKGROUND OF THE INVENTION

The invention relates to processes and apparatuses for separation ofgaseous components by means of selective and adiabatic adsorption anddesorption of usually considered unwanted impurities, using suitableadsorbents. The processes concerned are commonly known under the namepressure swing adsorption. Many different systems have been described inthe patent literature, all characterized by the general feature thatremoval of at least one impurifying component is effected throughselective adsorption at high pressure by at least one type of adsorbent,densely packed in a pressure vessel, called adsorber. During theadsorption step, feed gas is introduced at the inlet end of theadsorber, producing what is called primary product at the outlet endthereof. Dependent on the feed gas composition, adsorbers may containmore than one type of adsorbent, packed in different vertical zones ontop of one another. Types of adsorbents, commonly used in the art mayinclude zeolitic molecular sieves, activated carbon, silica gel oractivated alumina.

The indications cocurrent and countercurrent, used hereafter in thedescription of the various process steps are related to the direction offeed gas flow inside the adsorbers during the adsorption step.

The adsorbents are regenerated by desorbing components throughcounter-current depressurization until the lowest pressure, producingwhat is called dump gas and by countercurrent purging at the lowestpressure with near product quality gas, producing purge offgas.

After adsorption, additional near product quality gas of lower pressureis recovered, hereafter called secondary product gas, by depressurizingthe adsorber co-currently with the feed inlet end closed. This gasoriginates from the total void space in the adsorber and fromfractionated desorption from adsorbents therein. Production of saidsecondary product gas by said cocurrent depressurization is madepossible without a significant breakthrough of an adsorption front of anunwanted component, provided the adsorption step is ended early enough.

Final depressurization takes place countercurrently down to the lowestpressure, thereby releasing said dump gas. Such dump gas consists of atleast one desorbed component in admixture with some of the productcomponent.

The purging of the adsorbent takes place at the preferably lowestpressure by a countercurrent flow through it of purge gas, being a partof said secondary product gas, which thereafter, enriched with at leastone desorbed component is collected as said purge offgas.

The combined streams of dump gas and purge offgas are discarded asoffgas. Repressurization of the adsorber is realized with its inlet endclosed, by admission through its outlet end of, (1) the remaining partof said secondary product gas for the hereinafter as such indicated lowlevel repressurization and finally (2) a part the high pressure productgas, available as a split-off thereof, for the hereinafter as suchindicated high level repressurization. On reaching the highest pressure,the contained regenerated adsorbents are ready to undergo a newadsorption step.

According to U.S. Pat. No. 3,430,418 to J. L. Wagner, a minimum of fouradsorbers is required for a continuous operation, without requiringadditional gas storage vessels, such that always at least one of them isused for adsorbing impurities from feed gas, while the adsorbents in theremaining adsorber or adsorbers are undergoing the other process stepsof cocurrent and/or countercurrent depressurization, purging andrepressurization.

For a given set of process conditions of feed gas composition, feed gaspressure and desirable offgas pressure, while aiming for a maximumproduct recovery efficiency, a certain optimum can be established withrespect to the distribution of the secondary product gas over its reusefor purging and for low level repressurization. Maximum product recoveryefficiency is consistent with the lowest possible concentration of theproduct component in the offgas, a condition which is met by using arestricted amount of purge gas. If the restricted amount of purge gas issufficient for an adequate degree of adsorbent regeneration, any amountof available secondary product gas in excess of said restricted amountof purge gas should be used in a useful manner. It is one of thesubjects of this invention to improve the utilization of said excess forlow level adsorber repressurizations.

The effect of using more or less purge gas as clarified by the diagramof FIG. 1, showing the molar concentration of product component in theoffgas each adsorber produces during dumping and purging. The value “C”of said concentration is plotted versus the percentage “Q” of the totalof such offgas. The composition of the front end of the dump gas, shownat the left hand side of said diagram, is always identical to thecomposition of the feed gas; as it originates from the void spaces ofthe inlet end of an adsorber and its connected piping. While dumpingcontinues towards a continuously dropping pressure, the adsorbent's voidspace releases gas which due to desorption contains more and moreimpurities, causing the value C to drop. Upon reaching the purgingpressure, dumping is stopped. At this point Qd percent of the totaloffgas which an adsorber releases during a cycle has been produced. Theamount indicated between Qd and 100% represents the purge offgas, theconcentration of product component therein gradually rises until theavailable quantity of purge gas has been spent and the purging isfinished. This clarifies, that by using less purge gas, less purgeoffgas is produced, containing less product component as well,consistently resulting in a higher product recovery efficiency. However,a sufficient quantity of purge gas remains required for an effectiveregeneration of the adsorbents near the outlet end of the adsorbers andto realize a sufficiently large loading difference after the adsorptionstep consistent with a commercially acceptable utilization of theadsorbents.

By increasing the internal pressure recovery efficiency, defined aspressure rise realized by secondary product gas relative to the pressurerise with total repressurization, then a smaller complementary part ofsaid secondary product gas can be used for purging. One way of achievinga better control over the recovery and the distribution of secondaryproduct gas over low level repressurization and purging is described inU.S. Pat. No. 3,564,816 granted to L. B. Batta. According to thisdescription for a system consisting of 4 adsorbers, the internalpressure recovery efficiency is increased and the proportion of thetotal recovered quantity of secondary product gas reused for purging isreduced effectively with respect to realizing a higher product recoveryefficiency. Increase of the internal pressure recovery efficiency inthis system is realized by using the tail end of the released secondaryproduct gas for the initial low level repressurization of an adsorberinstead of for continuation of the purging of said adsorber.

Although this method, hereafter indicated as Batta-method, also whenused in systems with more than 4 adsorbers, increases the internalpressure recovery and as a result, the product recovery efficiency, theoffgas production is interrupted during said first stage ofrepressurizing, requiring large surge drums to dampen the irregularitiesof the offgas flow. Furthermore, an increase of the internal pressurerecovery may have a limited effect on the product recovery efficiency insuch cases, because for such increase, the following parameters arelikewise increased: (1) the start-of-dump pressure, (2) the quantity ofdump gas, (3) the content of the product component in the dump gas, andtherefore (4) product loss. In addition, any such breakthrough ofimpurities will be at its maximum at said increased start-of-dumppressure and since the recovered secondary product gas is used forcountercurrent low level repressurization of an adsorber, theseimpurities, due to the shortened range until the highest pressure isreached, will become re-adsorbed at a final position more close to theproduct end of said adsorber. Moreover, due to said shortened range, themagnitude of said re-adsorption will be less, leaving more of saidimpurities in the gaseous phase and therefore in the final product.Another way of increasing the internal pressure recovery efficiency andtherefore of reusing a lower complementary proportion of secondaryproduct gas for purging, is by increasing the number of participatingadsorbers in the process like as described in the aforementioned patentto Batta, using 5 adsorbers and as described in U.S. Pat. No. 3,986,849to A. Fuderer and E. Rudelsdorfer, using up to 10 adsorbers in thesystem. Each time when an adsorber is depresurrized by the cocurrentrelease of secondary product gas, such gas is distributed over more thanone adsorber until for each next adsorber receiving such gas at a lowerpressure level, equilibration is attained. In addition, the last portionof the released secondary product gas could be reused for low levelrepressurization instead of for purging as aforementioned for the Battainvention, however with the adverse effects of offgas flow interruptionand of the increased start-of-dump pressure.

Still another invention is described in U.S. Pat. No. 4,350,500 to A. J.Esselink on the improvement of the internal pressure recoveryefficiency, as is explained hereafter.

Because product gas is of the highest available pressure and of thehighest purity, final repressurization takes place by the productsplit-off only, hence by high level repressurization. However where theflow rate of the net product should be kept constant, said productsplit-off should be withdrawn therefrom without interruption, leading toa surplus thereof when said product split-off is not needed for finalhigh level repressurization. In cases where no useful purpose for saidsurplus other than for repressurization can be considered, said surplusis combined with said secondary product gas which is used for partiallow level repressurization until pressure equilibration between twoadsorbers, resulting into a higher equilibration pressure than if onlysecondary product gas had been used until such pressure equilibration.Contrary to the above and in accordance with the aforementioned Esselinkinvention, combining said product split-off and said secondary productgas is avoided in systems where at least two adsorbers are or will be inthe condition of receiving feed gas and where by delaying the moment ofswitching on a next adsorber to adsorption, the final high levelrepressurization of this adsorber is likewise made to continue to befinally repressurized by said product split-off in a last stage, therebyeliminating said surplus. Since said product split-off no longerinterferes with the pressure equilibration between two adsorbers, theequilibration pressure between said two adsorbers will be lower, causingthe remaining part of secondary product to be used for purging to belower as well. The number of adsorbers L, receiving feed gas in parallelwill be at least two, whereby said number is temporarily reduced to L−1after switching off one of these while the switching on of a regeneratedadsorber to adsorption is delayed. During said delay said regeneratedadsorber remains to be subjected to high level repressurization whileconsequently the velocity of the feed gas in the adsorber or adsorbersbeing switched on but as such in number been reduced by one, istemporarily increased by the ratio L/(L−1), provided the total stream offeed gas remains unchanged; the consequence of using this method ofreducing the quantity of purge gas is, that the feed gas velocity in anadsorber for a fixed quantity of feed gas per adsorber varies inaccordance with said ratio. Said variance is permitted as explainedhereafter.

During adsorption, adsorption and desorption fronts are establishedinside the adsorbers concerned, marking the extent by which adsorbablecomponents in the gaseous phase are carried along cocurrently with feedgas. Behind said fronts are active zones, known as mass transfer zones,where exchange of adsorbable components between the gaseous and adsorbedphase take place. The heights of these mass transfer zones depend (1),on diffusion parameters, affecting the resistance to mass transfer withrespect to the exchange of adsorbable components between the gaseous andadsorbed phase, (2), on the driving forces for adsorption anddesorption, and (3), on the feed gas velocity. Increasing or decreasingthe feed gas velocity has a corresponding effect on the height of themass transfer zones.

To achieve an efficient utilization of the adsorbent, the heights of themass transfer zones should be small at the moment in time whenadsorption and desorption fronts are no longer advancing, which is atthe end of the cocurrent depressurization. Well ahead of said moment,the velocity of the cocurrent gas flow should be kept small, such thatthe heights of the mass transfer zones are allowed sufficient time tobecome correspondingly small towards said moment, as if no prior periodsof higher gas velocities had existed. Because gas velocities duringcocurrent depressurization will be almost zero near to the closed feedinlet end, the period for lower gas velocities affecting adsorption anddesorption fronts, if more near to said feed inlet end, shouldpreferably be chosen well ahead of the start of the cocurrentdepressurization and therefore while still in the adsorption stage.

Therefore, without effect on the final adsorber conditions with respectto adsorbent loadings and adsorbate distribution between gaseous andsolid phases, the adsorber feed gas velocity may be higher during acertain interval of time, provided said velocity is reduced again to alower level, in general well ahead of the end of the adsorption step.

SUMMARY OF THE INVENTION

Surprisingly, it has been found that the internal pressure recoveryefficiency can be significantly improved by implementing, during a partof said delay, an additional pressure equilibration, between saidregenerated adsorbed and said switched off adsorber while the latterreleases secondary product gas in a first stage, supplementary to theproduct split-off for repressurizing said regenerated adsorber. The highlevel repressurization is thereby divided into two stages, as such beingconsidered a part of this invention. By practicing this invention undercircumstances where the increase of the internal pressure recovery isbeneficial, a significant improvement of the product recovery efficiencyof 2% to 3% becomes feasible.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Brief Description of the Drawings:

FIG. 1 is a concentration diagram according to the prior art;

FIGS. 2 and 3 comprise cycle diagrams according to the prior art;

FIGS. 4-6 comprise cycle diagrams according to an embodiment of thepresent invention;

FIGS. 7-9 are concentration diagrams according to the present invention;

FIG. 10 is a schematic diagram illustrating one embodiment of thepresent invention;

FIG. 11 is a cycle diagram according to another embodiment of theinvention;

FIG. 12 is a schematic diagram illustrating another embodiment of theinvention;

FIG. 13 is a cycle diagram illustrating yet another embodiment of theinvention;

FIG. 14 is a schematic diagram illustrating yet another embodiment ofthe invention;

FIG. 15 is a cycle diagram illustrating yet another embodiment of theinvention;

FIG. 16 is a schematic diagram illustrating yet another embodiment ofthe invention;

FIG. 17 is a cycle diagram illustrating yet another embodiment of theinvention;

FIG. 18 is a schematic diagram illustrating yet another embodiment ofthe invention;

FIGS. 19a. and 19 b. are cycle diagrams illustrating yet anotherembodiment of the invention;

FIG. 20 is a schematic diagram illustrating yet another embodiment ofthe invention;

FIGS. 21a. and 21 b. are cycle diagrams illustrating yet anotherembodiment of the invention;

FIG. 22 is a schematic diagram illustrating yet another embodiment ofthe invention;

FIGS. 23a. and 23 b. are cycle diagrams illustrating yet anotherembodiment of the invention;

FIG. 24 is a schematic diagram illustrating yet another embodiment ofthe invention;

FIGS. 25a. and 25 b. are cycle diagrams illustrating yet anotherembodiment of the invention;

FIG. 26 is a schematic diagram illustrating yet another embodiment ofthe invention;

FIGS. 27a. and 27 b. are cycle diagrams illustrating yet anotherembodiment of the invention;

FIGS. 28a. and 28 b. are cycle diagrams illustrating yet anotherembodiment of the invention;

FIG. 29 is a schematic diagram illustrating yet another embodiment ofthe invention;

FIG. 30 is a cycle diagram illustrating yet another embodiment of theinvention;

FIG. 31 is a cycle diagram illustrating yet another embodiment of theinvention;

FIG. 32 is a cycle diagram illustrating yet another embodiment of theinvention;

FIG. 33 is a schematic diagram illustrating yet another embodiment ofthe invention;

FIG. 34 is a cycle diagram illustrating yet another embodiment of theinvention; and

FIG. 35 is a cycle diagram illustrating yet another embodiment of theinvention.

The improvements and advantages of this invention and the differenceswith prior art are explained by comparing internal pressure recoveriesof the following five examples.

In said five examples, pressure versus time diagrams are shown, eachcovering one cycle element. Instead of covering a time span over acomplete cycle, such cycle is cut into a number of identical cycleelements equal to the number of adsorbers. In said diagram all theconsecutive functions of adsorbers within the time span of a cycleelement are shown, by tracking a line representing some functionstarting at the left to the right and then re-entering the diagram againat the left at the same pressure, whereby the new line represents eitherthe next or the same function as the case may be.

EXAMPLE 1

FIG. 2, representing one cycle element, shows the pressures in apressure swing adsorption unit of prior art. The proper sequence of theconsecutive functions for each adsorber is: A1, A2, D1, D2, PS, Du, P,R2, R1P, RP, etc. Specifically, said functions are, A1 and A2 foradsorption, three for cocurrent depressurization, (1): D1, equilibratingwith R1P at a first equilibrium pressure, (2): D2, equilibrating with R2at a second equilibrium pressure, (3): PS, for purgegas supply to purgeP, one for dumping Du, one for purging P, and three forrepressurization, (1): R2, equilibrating with D2 at said secondequilibrium pressure, followed by isolation until (2): R1P, low levelrepressurization, equilibrating with D1 at said first equilibriumpressure, combined with high level repressurization, (3): RP: high levelrepressurization until the highest pressure is achieved, after whichfunction A1 starts. The position in the diagram where D1 and R1P arereaching pressure equilibrium and where RP, the final high levelrepressurization should start, is indicated by the term SLDF. Itindicates the fraction of the cycle element time during which low leveland high level repressurization are taking place simultaneously.

EXAMPLE 2

FIG. 3, representing one cycle element, shows the pressures in apressure swing adsorption unit, also of prior art with an increasedinternal pressure recovery, similar to the system as described for fouradsorbers in the aforementioned Batta patent. The sequence of theconsecutive functions for each adsorber is: A1, A2, D1, D2, PS, D3, Du,P, R3, R2, R1P, RP, etc. Specifically, said functions are, A1 and A2 foradsorption, four for cocurrent depressurization, (1): D1, equilibratingwith R1P at a first equilibrium pressure, (2): D2, equilibrating with R2at a second equilibrium pressure, (3): PS, for purge gas supply to purgeP, (4): D3, equilibrating with R3 to a third equilibrium pressure, onefor dumping Du, one for purging P, and four for repressurization, (1):R3, equilibrating with D3 to said third equilibrium pressure, (2): R2,equilibrating with D2 to said second equilibrium pressure, followed byisolation until (3): R1P, low level repressurization and equilibratingwith D1 at said first equilibrium pressure combined with high levelrepressurization and (4): RP, final high-level-only-repressurization,until the highest pressure is attained, after which function A1 starts.During the low level repressurization R3 by D3, one of the typicalprocess steps according to the Batta invention, the offgas flow isinterrupted.

EXAMPLE 3

In this example, the internal pressure recovery is increased by thereduction by one of the number of adsorbers being simultaneouslysubjected to adsorption. The parameter by which this process variable isdescribed, is defined as sorption modulation, hereinafter indicated bythe term SM. The magnitude of this parameter represents the fraction ofthe cycle element time during which said reduction is applied.

A cycle element in FIG. 4 shows all the consecutive functions of whichthe proper sequence is: A1, A2, D1, D2, PS, Du, P, R2, R1, RP, etc.Specifically, said functions are, A2 for adsorption during the cycleelement fraction SM, followed by A1 and A2 for adsorption, three forcocurrent depressurization, (1): D1, equilibrating with R1 at a firstequilibrium pressure, (2): D2, equilibrating with R2 at a secondequilibrium pressure, (3): PS for purge gas supply to purge P, one fordumping Du, one for purging P and three for countercurrentrepressurization (1): R2, equilibrating with D2 to said secondequilibrium pressure, followed isolation until the end of the cycleelement, (2): R1, equilibrating with D1 at said first equilibriumpressure, possibly followed by a brief isolation, (3): RP, high levelrepressurization, until the highest pressure is attained at the cycleelement fraction SM, after which function A1 starts.

EXAMPLE 4

Under the condition where sorption modulation is applied, a furtherincrease of the internal pressure recovery is realized by an additionalpressure equilibration. The parameter by which this process variable isdescribed, is indicated hereinafter by the term SMEQ. The magnitude ofthis parameter represents the fraction of the cycle element time duringwhich the adsorber, after a period of only high level repressurization,in addition at the start of the next cycle element is subjected to lowlevel repressurization until pressure equilibration. After this, thehigh-level-only-repressurization is continued until the highest pressureis attained at the value SM. Consequently, thehigh-level-only-repressurization is divided into two stages.

A cycle element in FIG. 5 shows all the sequential functions for eachadsorber: A1, A2, D1, D2, D3, PS, Du, P, R3, R2, RP, R1P, RP, etc.Specifically, said functions are, A1 and A2 for adsorption, four forcocurrent depressurization (1): D1, equilibrating with R1P at a firstequilibrium pressure, (2): D2, equilibrating with R2 at the secondequilibrium pressure, (3): D3, equilibrating with R3 at the thirdequilibrium pressure, (4): PS, for purge gas supply to purge P, one fordumping Du, one for purging P, five for repressurization, (1): R3,equilibrating with D3 at said third equilibrium pressure, (2): R2,equilibrating with D2 at said second equilibrium pressure, followed byisolation until cycle element fraction SM, (3): RP,high-level-only-repressurization for the remaining part of the cycleelement, (4): R1P, equilibrating with D1 at said first equilibriumpressure at the value SMEQ, combined with high level repressurization,(5): RP, the final high-level-only-repressurization until the highestpressure is attained at the value SM, after which function A1 starts.

EXAMPLE 5

This example is a combination of examples 2 and 4, using the increasedinternal pressure recovery similar to the aforementioned Batta inventionbased on four adsorbers and as explained in example 2 and the additionalincrease of the internal pressure recovery according to this inventionand as explained in the previous example 4.

A cycle element in FIG. 6 shows all the sequential functions for eachadsorber: A1, A2, D1, D2, D3, PS, D4, Du, P, R4, R3, R2, RP, R1P, RP,etc. Specifically, said functions are, initially A2 for adsorptionduring the cycle element fraction SM, followed by A1 and A2 foradsorption, five for cocurrent depressurization, i.e. (1): D1,equilibrating with R1P at cycle element fraction SMEQ at the firstequilibrium pressure, (2): D2, equilibrating with R2 at the secondequilibrium pressure, (3): D3, equilibrating at the third equilibriumpressure, (4): PS; fur purge gas supply; (5): D4, for equilibrating withR4 at the fourth equilibrium pressure, Du for dumping, P for purging,six for repressurization, i.e. (1), R4, for equilibrating with D4 atsaid fourth equilibrium pressure, (2): R3, for equilibrating with D3 atsaid third equilibrium pressure, followed by isolation which continuesinto the next cycle element until the value SMEQ, (3): R2, equilibratingwith D2 at said second equilibrium pressure, followed by isolation untilthe value SM, (4): RP, the high-level-only-repressurization for theremaining part of the cycle element, (5): R1P, equilibrating with D1 atsaid first equilibrium pressure at the value SMEQ, combined with highlevel repressurization, (6): RP, the high-level-only-repressurizationuntil the highest pressure is attained at the value SM, after whichfunction A1 starts.

The internal pressure recoveries of the examples 1 to 5 can be comparedto one another on an equal basis by assuming empty adsorber vessels andneglecting adiabatic thermal effects. The following parameters aredefined, IPR: the internal pressure recovery, DPN: the nominal number ofpressure equilibrations, not including the implementation of theadditional pressure equilibration according to this invention, Pf: thefeed gas pressure, Pd: the start-of-dump pressure and Pp: the purgingpressure.

The percentage values for the internal gas recoveries can be formulatedfor the given examples, using the corresponding number of the followingequations, whereby all the pressures are quoted in kPa absolute:

1: IPR=100×(DPN−SLDF)/(DPN−SLDF+1)

2: IPR=100×[Pd−Pp+(Pf−Pd)×(DPN−SLDF)/(DPN−SLDF+1)]/(Pf−Pp)

3: IPR=100×DPN/(DPN+1)

4: IPR=100×(SM−SMEQ+DPN)/(SM−SMEQ+DPN+1)

5: IPR=100×[Pd−Pp+(Pf−Pd)×(SM−SMEQ+DPN)/(SM−SMEQ+DPN+1)]/(Pf−Pp)

Some results based on the examples 1 to 5 are summarized in Table 1,where n.r. stands for Not Relevant and n.a. for Not Applicable.

TABLE 1 Example Nr. 1 2 3 4 5 Equation Nr. 1 2 3 4 5 DPN 2 2 2 2 2 Pf(kPa) n.r. 2600 n.r. n.r. 2600 Pd (kPa) n.r. 450 n.r. n.r. 450 Pp (kpa)n.r. 150 n.r. n.r. 150 SLDF/SMEQ (−) 0.18 0.20 0.00 0.15 0.15 SM (−)n.a. n.a. 0.60 0.75 0.75 Offgas Interruption no yes no no yes IPR (%)64.54 68.66 66.67 72.22 75.62

Comparing examples 1 and 4, as well as examples 2 and 5, learns thatintroducing an additional pressure equilibration in accordance with thepresent invention results into a substantial increase of the internalpressure recovery. The differences between examples 1 and 2, as well asbetween examples 4 and 5, showing the contribution in terms of internalpressure recovery in accordance with the aforementioned Batta invention,appear to be considerably less and in terms of offgas continuity and ofhigher start-of-dump pressures also less attractive. Example 3 refers tothe aforementioned Esselink invention using the principle of sorptionmodulation. Sorption modulation SM as applied in the preceeding example3, with or without an additional pressure equalization SMEQ as appliedin the preceding examples 4 and 5, requires a minimum of two adsorberspotentially being simultaneously subjected to adsorption. Depending onthe required capacity of a pressure swing adsorption unit the choice onthe total number of adsorbers may be 4 for the smaller, and up to 12 forthe largest units. If the value of SM approaches 1, either with orwithout option SMEQ, the number of adsorbers actually being subjected toadsorption likewise approaches the condition of remaining unchanged. Inany case, using option SMEQ means the addition of an extra pressureequilibrium.

Purging is most effective at low pressure, facilitating desorption andincreasing the sweeping volume of the purge gas. However, the selectionof a low purging pressure is not advantageous if the resulting lowpressure offgas cannot be used directly in the usual way as fuelgas butneeds to be compressed before it can be admitted into an existing higherpressure fuelgas grid, therefore, in order to avoid the necessity ofcompressing the offgas, a higher purging pressure may be selected.

A higher purging pressure has a number of consequences, like, a higherstart-of-dump pressure, a shorter range for cocurrent relativedepressurization, a lower degree of desorption at the start and at theend of dumping, a lower concentration of the impurities in the gaseousphase in the adsorbent void space and complementary to this a higherconcentration therein of the product component and a higher loss of saidproduct component during the subsequent dumping step, as well as duringthe purging step thereafter.

The effect of a higher purging pressure is shown by comparing the twodiagrams of the FIGS. 1 and 7, giving the offgas profiles as explainedabove with respect to FIG. 1. The diagram of FIG. 7 represents a case ofa relatively high purging pressure. On closer examination of thecomposition profiles of dump gas and purge offgas and of the effect of ahigher purge pressure, it turns out that the minimum value of theconcentration “C” of the product component appears to be higher, whilethis concentration appears to be lower in the tail end of the offgas.This more or less flat profile of the purge offgas is due to the lowerdegree of desorption.

Surprisingly, it has been found that the product recovery efficiencywhen using a relatively high purge gas pressure, may be increased byadmitting dump gas into a regenerated adsorber via its inlet end withits outlet end being closed, and so realizing initial repressurization.Said repressurization takes place in cocurrent instead of in the usualcountercurrent direction, possibly until pressure equilibration isreached. By using this technique, hereinafter indicated as dump return,the part of dump gas with the highest concentration of product componentis returned into the system instead of being released as part of theoffgas. For achieving an improvement of the product recovery efficiency,it is necessary that the average concentration of product component inthe part of dump gas being returned should be noticeably higher than inthe remaining part of the total of dump gas and purge offgas. Theconcentration of the product component in the tail end part of the purgeoffgas should have a rather flat profile and should not be appreciablyhigher than its minimum in the offgas.

The effect of dump return on the product recovery efficiency in anegative sense, but also in a positive sense can be clearly demonstratedby comparing to one another the diagrams of FIGS. 1 and 8. In case of arelatively low purge pressure, the FIGS. 1 and 8 may serve to explainsaid effect in the negative sense. The diagram of FIG. 8 is almostequivalent to the diagram of FIG. 1, except for the absence of thereturned part of the dump gas with a relatively low percentage ofproduct component and equal to Qr % of the total offgas of FIG. 1. Dueto the very minor effect of dump return on the composition of theremainder of the offgas, the net effect will be as represented by FIG.8, consistent with an average higher percentage of product component inthe total offgas and therefore with the lower product recoveryefficiency.

The FIGS. 7 and 9 show offgas profiles when using a relatively highoffgas pressure, FIG. 9 presenting the profile of the net offgas aftersubtracting the recycled part of the dump gas, equal to Qr% of theoriginal offgas of FIG. 7. Dump return in the latter case results into alower average percentage of product component in the net offgas andtherefore into a positive effect on the product recovery efficiency.Whereas the product recovery efficiency may be improved by 1 to 2percentage points by using dump return, an additional improvement of 1to 1.5 percentage point is achieved if the use of sorption modulation isincluded.

In accordance with prior art, the adsorber product outlets in pressureswing adsorption plants are connected with a product collecting header,each via an on/off product collecting valve. The aforementioned productsplit-off is withdrawn from said product collecting header and via acontrol valve metered into a second header, which has been provided withseparate control valves each of them being connected with an adsorberproduct outlet, such that the product split-off can be admitted to anadsorber for high level repressurization. Simultaneously, said secondheader may serve to carry secondary product gas for low levelrepressurization of an adsorber until pressure equalization has beenattained. Therefore, said separate control valves are provided with thenecessary means to control their position and with it the flow ofsecondary product gas through them.

Unexpectedly, it has been found that if, in contrast to said prior art,the product split-off is metered directly from the product collectingheader into the product end of an adsorber undergoing high levelrepressurization, then, (1) the minimum number of required valves isreduced and (2) for plants comprising an assembly of at least sevenadsorbers, at least one additional pressure equilibration under theoption SMEQ may be realized without requiring additional valves oversaid reduced minimum number mentioned under (1) above. Said metering ofproduct split-off is realized in countercurrent direction through saidproduct collecting valves in an on/analog/off control function.

An advantage of the provision of an analog control in addition to theon/off function of said product collecting valves, is the possibility ofcontrolling through them the distribution of the constant feed gas flowover adsorbers being subjected to adsorption simultaneously, such thatthe feed gas velocity inside one of said adsorbers if wanted forachieving a higher loading of the adsorbents therein, may be reduced fora while towards the end of the adsorption step thereof, while the feedgas velocity in the other adsorber or adsorbers being subjected toadsorption simultaneously is permitted to be higher.

It will be clear for those who are familiar with the technicalrealization of the pressure swing adsorption process, that many ways arepossible for application of the principle of sorption modulation,especially if option SMEQ is included. In this respect and morespecifically, different possible embodiments of the invention areexplained, using illustrations depicting the arrangement of adsorbersand the necessary piping and valves for directing the various gas flows.As a common characteristic for sorption modulation option SMEQ, thenumber of adsorbers being simultaneously switched on for adsorption varybetween L and L−1 whereby, dependent on the unit, L may be minimum two,to maximum four. That further a first adsorber is subjected to highlevel repressurization: in three phases, (1) between the value SM andthe end of a cycle element, (2) between the start and the value SMEQ ofthe next cycle element, combined with a low level repressurization bysecondary product in a first step produced by a second adsorber upon itsswitching off from adsorption, the latter coinciding with the start ofsaid next cycle element, thereby reducing by one the number of adsorberssimultaneously participating in adsorption, further (3) until thehighest pressure is attained at the value SM of said next cycle elementand said first adsorber is switched on for adsorption, therebyincreasing by one the number of adsorbers simultaneously participatingin adsorption. Along with these process steps an additional pressureequilibration is implemented, resulting in an increased product recoveryefficiency. Next to the illustrations depicting the arrangement ofadsorbers, piping and valves, diagrams show schematically adsorberpressures versus time, covering one complete cycle. The characters andassociated numbers, attached to the lines in said diagrams, serve toidentify the valves which are open for gas flow control during theindicated time periods, the characters refer to the correspondingadsorbers, and storage vessels where applicable.

FIG. 10 shows the embodiment consisting of an assembly of fouradsorbers, in alphabetical order indicated by A to D, and a storagevessel E. The secondary product gas after partly being used forrepressurization is collected and temporarily stored in vessel E untilits use as purge gas. After dumping until the lowest pressure isattained follows purging. The number of pressure equilibrations betweenadsorbers is two. The third pressure equilibration takes place betweenstorage vessel E and an adsorber releasing a third and last portion ofsecondary product gas. The start-of-dump pressure equals the level ofsaid third pressure equilibration. FIG. 11 shows the pressure versustime diagram with 6 periods per cycle element. The provision of theadditional pressure equilibration is shown in the periods 1, 7, 13, 19,etc., each time dividing the high level repressurization into threestages, whereby during the second stage in addition a low levelrepressurization, said additional pressure equilibration is effected.

FIG. 12 shows the embodiment consisting of an assembly of fiveadsorbers, in alphabetical order indicated by A to E, and a storagevessel F. The secondary product gas after partly being used forrepressurization is collected and temporarily stored in vessel F untilits use as purge gas. After-dumping until the lowest pressure isattained follows purging. The total number of pressure equilibrationsbetween adsorbers is three. The fourth pressure equilibration takesplace between storage vessel F and an adsorber releasing a fourth andlast portion of secondary product gas. The start-of-dump pressure equalsthe level of said fourth pressure equilibration. FIG. 13 shows thepressure versus time diagram with 6 periods per cycle element. Theprovision of the additional pressure equilibration is shown in theperiods 1, 7, 13, 19, 25, etc., each time dividing the high levelrepressurization into three stages as in the preceding case for fouradsorbers. In TABLE 2, the IPR-values for systems consisting of four andfive adsorbers are compared to one another, comprising theaforementioned inventions by Wagner and Batta and this subjectinvention, showing the improvement for subject invention as described inthe FIGS. 10 and 11 for four adsorbers and in the FIGS. 12 and 13 forfive adsorbers. It should be noted that the use of sorption modulationin a system consisting of four and five adsorbers a vessel is requiredfor the intermediate storage of secondary product gas for subsequent useas purge gas, in the FIGS. 12 and 13 indicated by vessel E and by vesselF respectively.

An important aspect of the already aforementioned Esselink invention inU.S. Pat. No. 4,350,500 concerns the storage of secondary product in oneor more pressure vessels, filled with a nonporous inert packing with ahigh void fraction, as may be achieved with thin walled metal Raschigrings. Aim of a packing like this is, to preserve the compositionprofile during said storage by avoiding internal mixing so that on reuseof this gas for the purging of adsorbents and/or the repressurization inan adsorber, the fraction with the highest percentage of impurities,corresponding with the last part recovered secondary product, is used asfirst part for said reuse. In order to achieve that according to theprocesses of FIG. 10 and FIG. 12, the sequence of recovery and storageof secondary product in the pressure vessels E, and F respectively, isreversed to the sequence of its reuse as purge gas, said pressurevessels are filled with said packing. In this way, it is achieved thatduring purging the percentage of impurities in the purge gas drops, i.e.the quality is improved, in the same way this percentage increasesduring its recovery and storage. Through the possibility of qualityimprovement of the purge gas during purging, a higher product purity isachieved, at unchanged remaining process conditions.

TABLE 2 Invention Wagner Batta Subject Batta Subject Equation Nr. 1 2 41 4 Number of 4 4 4 5 5 adsorbers DPN 1 1 1 2 2 Pf (kPa) n.r. 2600 n.r.n.r. n.r. Pd (kPa) n.r. 450 n.r. n.r. n.r. Pp (kPa) n.r. 150 n.r. n.r.n.r. SLDF/SMEQ (−) 0.20 0.20 0.15 0.20 0.15 SM (−) n.a. n.a. 0.55 n.a.0.75 Offgas Interruption no yes yes no yes IPR (%) 44.4 51.2 58.3 64.372.2

FIG. 14 shows the embodiment consisting of an assembly of six adsorbersin alphabetical order indicated by A to F. The number of pressureequilibrations is three. Compared to the assemblies of four and fiveadsorbers, subject assembly of six adsorbers does not need a vessel forthe temporary storage of purge gas, and the offgas is not interrupted.FIG. 15 shows the pressure versus time diagram with 5 periods per cycleelement. The provision of the additional pressure equilibration is shownin the periods 1, 6, 11, 16, 21, 26, etc., each time dividing the highlevel repressurization into three stages like for instance in the abovecase comprising four adsorbers. The effect of the additional pressureequilibration, being an increased IPR-value is shown in Table 1 bycomparing to one another the examples 1 and 4. A similar effect resultsfor a six-adsorber-system using the Batta-method as shown by comparingto one another the examples 2 and 5.

FIG. 16 the embodiment is shown, consisting of an assembly of sevenadsorbers in alphabetical order indicated by A to G. The nominal plusthe additional number of pressure equilibrations is three. Compared tothe assembly of six adsorbers of FIG. 14, more time in this subject caseis taken for depressurization and purging. FIG. 17 shows the pressureversus time diagram with 4 periods per cycle element. The provision ofthe additional pressure equilibration is shown in the periods 1, 5, 9,13, 17, 21, 25, etc., each time dividing the high level repressurizationinto three stages. Based on the use of the for this case applicableequation 4 for sorption modulation, the IPR-value is: 72.1%, for DPN=2,SM=78% and SMEQ=20%. Compared to prior art in accordance with theaforementioned U.S. Pat. No. 3,986,849, using a 7-adsorber system withDPN=3, not using sorption modulation and not comprising an additionalpressure equilibration, the applicable equation 1 reveals an IPR-valueof: 71.4% for SLDF=50%. Although the IPR-values are rather similar, thedisadvantage of the prior art invention is however that the purge gashas to be withdrawn from the product and that a higher start-of-dumppressure should be used, which leads to a lower product recoveryefficiency. An alternative system based on seven adsorbers, not havingthe disadvantage of the purge gas being withdrawn from the product andnot with the necessity of having to use a higher start-of-dump pressure,is characterized by a lower IPR-value of: 63.0, based on DPN=2 andSLDF=30%, based on the for this case applicable equation 1.

FIG. 18 the embodiment is shown of an assembly of eight adsorbers inalphabetical order indicated by A to H. The number of pressureequilibrations is four, however at a fixed start-of-dump pressure. Thelatter is due to the fact that the last fraction of secondary productgas, following its use for purging, and prior to dumping, is used forthe first stage of repressurizing a regenerated adsorber until pressureequilibration. The start-of-dump pressure is thereby fixed at the levelof said pressure equilibration. FIG. 19 shows the pressure versus timediagram with 4 periods per cycle element. The provision of theadditional pressure equilibration is shown in the periods 1, 5, 9, 13,17, 21, 25, 29, etc., each time dividing the high level repressurizationinto three stages. Based on the applicable equation 5 for sorptionmodulation, the IPR-value is:76.0%, for DPN=2, SM=80%, SMEQ=15%, Pf=2600kPa, Pd=450 kPa and Pp=150 kPa. This compares very favorably to theprior art as described in the aforementioned U.S. Pat. No. 3,986,849,for which the applicable equation 2 reveals an IPR-value of 64.9% forDPN=2 and SLDF=50% and based on unchanged pressures.

FIG. 20 shows the embodiment consisting of an assembly of eightadsorbers in alphabetical order indicated by A to H. Compared to theconfiguration of FIG. 18, also consisting of eight adsorbers andemploying four pressure equilibrations, in this assembly, the lastfraction of secondary product gas is used for purging instead of forbeing used for low level repressurization and therefore leaving thepossibility for selecting the start-of-dump pressure independently fromthe other process parameters. However, this assembly requires one ormore valves per adsorber. FIG. 21 shows the pressure versus time diagramwith 4 periods per cycle element. The provision of the additionalpressure equilibration is shown in the periods 1, 5, 9, 13, 17, 21, 25,29, etc., each time dividing the high level repressurization into threestages. Based on the use of equation 4 for sorption modulation, theEPR-value is: 78.3%, for DPN=3, SM=80% and SMEQ=20%. Comparing to priorart in accordance with the aforementioned U.S. Pat. No. 3,986,849, usingan 8-adsorber system with DPN=4, not using sorption modulation and notcomprising an additional pressure equilibration, for this caseapplicable equation 1 reveals an IPR-value of: 77.8% for SLDF=50%.Although the EPR-values are rather similar, the disadvantage of theprior art invention is however that the purge time is only half of thecycle element time and also only half of the purge time of thisinvention, which could make the purge step less effective.

FIG. 22 shows the embodiment consisting of an assembly of nine adsorbersin alphabetical order indicated by A to I. Compared to the assembly ofeight adsorbers of FIG. 20, subject assembly of nine adsorbers employsthe same number of four pressure equilibrations, the difference beingthat in this subject case the maximum number of adsorbers simultaneouslybeing subjected to adsorption is three instead of two. FIG. 23 shows thepressure versus time diagram with 4 periods per cycle element. Theprovision of the additional pressure equilibration is shown in theperiods 1, 5, 9, 13, 17, 21, 25, 29, 33, etc., each time dividing thehigh level repressurization into three stages. Based on the use ofequation 4 for sorption modulation, the IPR-value is: 78.3%, for DPN=3%,SM=80% and SMEQ=20%. Comparing to prior art in accordance with theaforementioned U.S. Pat. No. 3,986,849, using a 9-adsorber system withDPN=3, not using sorption modulation and not comprising an additionalpressure equilibration, the for this case applicable equation 1 revealsan IPR-value of: 71.4% for SLDF=50%.

FIG. 24 shows the embodiment consisting of an assembly of ten adsorbersin alphabetical order indicated by A to J. Compared to the assembly ofnine adsorbers of FIG. 22, subject assembly of ten adsorbers employs thesame number of four pressure equilibrations, the difference being thatin this subject case the time period taken for the recovery and use ofpurge gas is about 50 percent longer, while the required number ofvalves per adsorber is reduced by one. The number of adsorberssimultaneously being subjected to adsorption varies between two andthree. FIG. 25 shows the pressure versus time diagram with 4 periods percycle element. The provision of the additional pressure equilibration isshown in the periods 1, 5, 9, 13, 17, 21, 25, 29, 33, 37, etc., eachtime dividing the high level repressurization into three stages. Basedon the use of equation 4 for sorption modulation, the IPR-value is:78.3%, for DPN=3, SM=80% and SMEQ=20%. Comparing to prior art inaccordance with the aforementioned U.S. Pat. No. 3,986,849, using a10-adsorber system with DPN=3, not using sorption modulation and notcomprising an additional pressure equilibration, the applicable equation1 reveals an IPR-value of: 71.4% for SLDF=50%.

FIG. 26 shows the embodiment consisting of an assembly of twelveadsorbers in alphabetical order indicated by A to L. Compared to theassembly of ten adsorbers of FIG. 24, subject assembly of twelveadsorbers employs five instead of four pressure equilibrations, thedifference being that in this subject case the number of adsorberssimultaneously being subjected to adsorption varies between three andfour instead of between two and three. FIG. 27 shows the pressure versustime diagram with 4 periods per cycle element. The provision of theadditional pressure equilibration is shown in the periods 1, 5, 9, 13,17, 21, 25, 29, 33, 37, 41, 45, etc., each time dividing the high levelrepressurization into three stages. Based on the use of equation 4 forsorption modulation, the IPR-value is: 82.2%, for DPN=4, SM=80% andSMEQ=18%. Comparing to prior art, using a 12-adsorber system, withDPN=4, not using sorption modulation and not comprising an additionalpressure equilibration, the applicable equation 1 reveals an IPR-valueof: 77.8% for SLDF=50%.

Using a different process, based upon the same embodiment of FIG. 26,the number of pressure equilibrations is six instead of five, while thenumber of adsorbers simultaneously being subjected to adsorption variesbetween two and three, instead of between three and four. FIG. 28 showsthe pressure versus time diagram with 4 periods per cycle element. Theprovision of the additional pressure equilibration is shown in theperiods 1, 5, 9, 13, 17, 21, 25, 29, 33, 37, 41, 45, etc., each timedividing the high level repressurization into three stages. Based on theuse of equation 4 for sorption modulation, the IPR-value is: 84.9%, forDPN=5, SM=78% and SMEQ=15%. Such a high IPR-value is advantageous when arelatively small quantity of purge gas is needed as in the case of ahigh ratio of feed gas pressure to purge gas pressure of at least 20,combined with a fairly high concentration of impurities in the feed gasof not less than 25 volume percent.

FIG. 29 shows the embodiment consisting of an assembly of six adsorbersin alphabetical order indicated by A to F, for using the combination ofsorption modulation options SM and SMEQ, and dump return. The totalnumber of pressure equilibrations is four, following three low levelrepressurization in countercurrent direction and one low levelrepressurization in cocurrent direction using dump return. FIG. 30 showsthe pressure versus time diagram with 5 periods per cycle element. Thetypical features of this embodiment comprise the combination of, (1) theprovision of the additional countercurrent pressure equilibration asshown in the periods 1, 6, 11, 16, 21, 26, etc., each time dividing thehigh level repressurization into three stages like among others in theprevious case, using four adsorbers and (2) the provision of low levelrepressurization by dump return as shown in the periods 3, 8, 13, 18,23, 28, etc.

Based upon the adsorber assembly of FIG. 29, the combination of sorptionmodulation option SM and dump return may be used, hence without theadditional pressure equilibration. Then the total number of pressureequilibrations is three, following low level repressurization, two incountercurrent direction and one in cocurrent direction using dumpreturn. FIG. 31 shows the pressure versus time diagram with five periodsper cycle element. The essential features of this variant comprise thecombination of, (1) sorption modulation with option SM only, where thehigh level repressurization remains completely separated from the lowlevel repressurization by the first part of secondary product gas in theperiods 1, 2, 6, 7, 11, 12, 16, 17, 21, 22, 26, 27, etc. and (2) theprovision of dump return in the periods 2, 7, 12, 17, 22, 27, etc. Dumpreturn may also be applied without sorption modulation as shown in FIG.32 with five periods per cycle element. Dump return takes place in theperiods during the periods 2, 7, 12, 17, 22, 27, etc. Said assembly ofsix adsorbers, if made suitable for implementation, of dump return, orof one of the two variants of sorption modulation, option SM, or optionsSM including SMEQ, either combined or separately, offers a high degreeof flexibility, with the possibility to switch between all these optionsduring operation, including the processes of prior art as described inthe above mentioned examples 1 and 2.

FIG. 33 shows a simplified arrangement of six adsorbers, whereby thenumber of valves is reduced by three and the use of sorption modulationis limited to option SM. Its reduced degree of flexibility stillcomprises all aforementioned possibilities of the arrangement asdepicted by FIG. 29, except sorption modulation SM combined with SMEQ.Said simplified arrangement requires a predetermined and fixed sequenceby which adsorbers undergo the process steps, which should appear aftercomparing to one another the FIGS. 34 and 31 and the FIGS. 35 and 32. Ineach of these comparisons, the valves A4 to F4 of FIG. 29 have beenreplaced by the valves AB4, CD4 and EF4. The following examples serve todemonstrate the benefits of this invention by comparing operatingresults therefrom to those based on prior art.

EXAMPLE 6

Gaseous hydrogen is recovered from a feed gas, produced by the steamreforming of naphtha and has the following specification. Composition involume percent is: H2:75.99, CO:2.69, CH4:3.58, CO2:17.50, H2O:saturated, at the current pressure of 2670 kPa and a temperature of 38.0deg. C. The pressure of the offgas is set at 150 kPa. The availablepressure swing adsorption unit, consists of six adsorbers has beenassembled as depicted by FIG. 14 and has been made suitable foroperation according to this invention as well as according to prior artin accordance with FIG. 2. At an internal diameter of the adsorbers of1800 mm, the adsorption zone has a total height of 6000 mm and consists,in the upper 18% part of zeolite molecular sieve of type 5A in the formof 1 to 2 mm granules without binder, the middle 73% part of activatedcarbon with an average particle diameter of 1.5 mm and the remaininglower part of narrow pore granulated silica gel with a diameter between1 and 3 mm. The performance of the unit is summarized in TABLE 3 below.

TABLE 3 Prior art This invention Feed gas rate (Nm3/h) 15075 15075 Cycletime (min.sec) 19.48 16.24 Product gas rate (Nm3/h) 10000 10218 Impurityin product, CO (ppm) 1 1 Product recovery efficiency (%) 87.3 89.2

Based on the same feed gas rate and product quality, it is demonstratedthat the recovery efficiency, using this invention is increased by 2.18%or 1.9 percentage points in comparison to prior art.

EXAMPLE 7

Gaseous hydrogen is recovered from a feed gas, a byproduct from an oilrefinery of the following composition in volume percent is: H2:66.205,CO:0.001, CH4:27.05, C2H6:6.60, C2H4:0.10, C6H6:0.043, C7H8:0.001, at apressure of 2368 kPa and a temperature of 17.3 deg. C. The offgas isproduced at the relatively high pressure of 420 kPa. The availablepressure swing adsorption unit, consisting of six adsorbers has beenarranged as depicted by FIG. 29 and has been made suitable for operationin accordance with each of the processes as indicated by the FIGS. 2, 3,4, 5 or 15, 6 and 30. At a diameter of the adsorbers of 1800 mm, theadsorbent beds in each of them has a height of 7500 mm and eachconsisting of 86% of activated carbon as granules of 1 to 2 mm near theoutlet part and small pore granulated silica gel with a diameter ofbetween 1 and 3 mm near the inlet part. The performance of the unit issummarized in TABLE 4 and is based upon prior art in accordance withFIG. 2 and upon the process in accordance with this invention, basedupon the use of sorption modulation SM including SMEQ and upon the useof dump return as depicted by FIG. 30.

TABLE 4 Prior art This invention according according Process: FIG. 2FIG. 30 Feed gas rate (Nm3/h) 15075 15075 Cycle time (min.sec) 19.4816.24 Product gas rate (Nm3/h) 10000 10218 SM (%) — 75 SMEQ (%) — 15Dump return (Nm3/step) — 100 Impurity in product, CH4 (vppm) 9810 9240C2H6 (vppm) 147 155 Product recovery efficiency (%) 80.30 82.90

Based on the same feed gas rate and approximately similar productquality, it is demonstrated that in comparison to prior art, therecovery efficiency, using this invention is increased by 3.2% or 2.6percentage points.

What is claimed is:
 1. A pressure swing adsorption process, comprisingefficiency improvements in a repetitive cycle, each cycle being dividedinto a number M of identical cycle elements and all functions occurringwithin each cycle element, the start and the end thereof coinciding withthe end of an adsorption function, a pressure swing process is carriedout for separation of gas mixtures by selective adsorption by at leastone adsorbent, in granular or pellet shaped evenly packed in and equallydistributed over an assembly of M′ pressure vessels called adsorbers,each provided with an inlet end and an outlet end and in number equal tosaid number M of said identical cycle elements, whereby each adsorber,starting at a fixed position in time in one of said cycle elements issubjected to a fixed sequence of functions, comprising (1)adsorption—extending itself over at least one cycle element—of at leastone component from a gas mixture, at a highest pressure andpredominantly existing of one or more of the least adsorbablecomponents, cocurrently flowing from the inlet part to the outlet partthereof, at said outlet part recovering primary product gas, (2) withits inlet end closed, cocurrent depressurization, starting at saidhighest pressure, producing at its outlet end secondary product gas ofnear primary product quality, by N−1 of N depressurization steps inbehalf of fractional repressurization, each time of an other adsorberthrough the outlet end thereof until pressure equilibrium is reached atthe end of each step reaching a lower pressure, and by one otherdepressurization step in behalf of the provision of purge gas forpurging at least one other adsorber, (3) with its outlet end closed,countercurrent depressurization until a lowest pressure is reached,thereby producing at its inlet end dump gas containing next to some ofthe least adsorbable component other, during this step, desorbedcomponents, (4) countercurrent purging with purge gas, flowing from theoutlet end to the inlet end thereof, thereby producing offgas whichcontains next to some least adsorbable component the remainder ofdesorbed components and whereby said purge gas is produced as secondaryproduct gas by another adsorber during one of its N cocurrentdepressurization steps, (5) with its inlet end closed, repressurizationby introducing via its opened outlet end, (5A) of secondary product gassimultaneously being produced by other adsorbers by N−1 cocurrentdepressurization steps until pressure equilibration is reached, eachtime at a higher pressure at the end of each of such steps and (5B) inat least one additional countercurrent admission step, by primaryproduct gas produced by at least one other adsorber, until the highestpressure is attained, the improvements (a) and (b), comprising: (a) anincrease of the product recovery efficiency in case at leasttemporarily, L adsorbers are parallel switched on adsorption functionand whereby each adsorption function takes more than one cycle elementand the number of countercurrent repressurization steps being one morethan the number of cocurrent depressurization steps such that,repressurization by a part of primary product gas takes place byadmitting this in at least three portions, one in each of the last threeof N+1 admission steps, (1) by only the first portion, during the secondbefore last admission step, between a time fraction SM of a cycleelement and the end thereof, (2) by the second portion together withsecondary product gas during the first before last admission step, thebeginning thereof coinciding (2A) with the transition to the next cycleelement, (2B) with the switching off of the adsorption function of another adsorber from adsorption, thereby decreasing with one the numberof adsorbers with switched on adsorption function, and wherefrom duringthe first of its N release steps said secondary product is produceduntil pressure equilibration is attained at the end of said first beforelast admission step, coinciding with the point in time of a fractionSMEQ of the duration of said next cycle element, (3) during the lastadmission step by only the third portion until the highest pressure isattained, coinciding with the fraction SM of the duration of said nextcycle element, such that the end of said last admission step coincideswith the second before last of the N+1 countercurrent repressurizationsteps of the next in the sequence of adsorbers and with the start of theadsorption step, increasing the number of adsorbers with switched onadsorption functions by one, and (b) an increase of the product recoveryefficiency, prior to said countercurrent repressurization, by onecocurrent repressurization step with its outlet end closed, throughadmitting the first part of dump gas produced by an other adsorber untilat most pressure equilibration is reached.
 2. The process according toclaim 1, wherein in improvement (a) M equals minimum 4 till maximum 5,N=M−1, L=2 and the installation of a pressure vessel for storage ofsecondary product gas is included, said gas originating from the last ofthe N cocurrent depressurization steps until pressure equilibration isattained, followed by release of said gas for its utilization as purgegas at the lowest pressure.
 3. The process according to claim 2, whereinsaid pressure vessel is filled with inert material in a loosely andevenly distributed packing of high void fraction.
 4. The processaccording to claim 1, wherein in improvement (b) M equals minimum 4 tillmaximum 5 and N=M−2.
 5. The process according to claim 1, wherein inimprovement (b) M equals minimum 6 till maximum 8, N=3 and two adsorbersare parallel switched on adsorption function.
 6. The process accordingto claim 1, wherein in improvement (b) M=8, N=4 and two adsorbers areparallel switched on adsorption function.
 7. The process according toclaim 1, wherein in improvement (b) M equals minimum 9 till maximum 10,N=4 and at least three adsorbers are parallel switched on adsorptionfunction.
 8. The process according to claim 1, wherein in improvement(b) M=12, N equals minimum 5 till maximum 6 and 9-N adsorbers areparallel switched on adsorption function.
 9. The process according toclaim 1, wherein in improvement (a) M equals minimum 6 till maximum 12,secondary product gas from the last of N depressurization steps is usedas purge gas, while the secondary product gas used for repressurizationand admitted with admission steps number 1 to N−2 is produced by thenumbers N−1 to 2 respectively of said N depressurization steps, afterwhich repressurization is continued with admission step N−1, exclusivelyby the first portion of the primary product split-off, followed withadmission step number N by secondary product gas from the first of saidN depressurization steps in admixture with the second portion of saidprimary product split-off till pressure equilibration is attained andfinally for admission step number N+1 exclusively by the third portionof said primary product split-off.
 10. The process according to claim 9,wherein M equals minimum 6 to maximum 7, N=4 and L=2.
 11. The processaccording to claim 9, wherein M=8, N=5 and L=2.
 12. The processaccording to claim 9, wherein M equals minimum 9 to maximum 10, N=5 andL=3.
 13. The process according to claim 9, wherein M=12, N equalsminimum 6 to maximum 7 and L=10-N.
 14. The process according to claim 1,wherein in improvement (a) M′ equals 6 or 8 or 10 adsorbers, secondaryproduct gas from the one before the last of N de-pressurization steps isused as purge gas, while the secondary product gas used forrepressurization and admitted with admission steps number 1 to N−2 isproduced by the numbers N, N−2 to 2 respectively of said Ndepressurization steps, after which repressurization is continued withadmission step N−1, exclusively by the first portion of the primaryproduct split-off, followed with admission step number N by secondaryproduct gas from the first of said N depressurization steps in admixturewith the second portion of said primary product split-off till pressureequilibration is attained and finally for admission step number N+1exclusively by the third portion of said primary product split-off. 15.The process according to claim 14, wherein M′=6, N=5 and L=2.
 16. Theprocess according to claim 14, wherein M′=8, N=5 and L=2.
 17. Theprocess according to claim 14, wherein M′=10, N=6 and L=3.
 18. Theprocess according to claim 1, wherein the least adsorbable component ishydrogen.
 19. The process according to claim 1, wherein the leastadsorbable component is nitrogen.
 20. The process according to claim 1,wherein the least adsorbable component is methane.
 21. The processaccording to claim 1, wherein SM is varied between 40 to 100% of theduration of a cycle element.
 22. The process according to claim 1,wherein SMEQ is varied between 5 and 50% of the duration of a cycleelement.
 23. The process according to claim 1, wherein primary productgas split-off, used for repressurization of each adsorber, is directlymetered from the product header through a control valve, directlyconnecting said adsorber with said product header.
 24. The processaccording to claim 23, wherein control valves, directly connecting anoutlet end of each adsorber with said product header, are used forcontrolling the distribution of feed gas over adsorbers with parallelswitched adsorption functions.
 25. The process according to claim 24,wherein distribution of feed gas is controlled such that its admissionspeed to an adsorber during a time span between the fraction SM and theend of the cycle element, said end coinciding with the end of theadsorption function thereof, is reduced to at least 25% of its maximumvalue.